Metallic Copper Species Research Articles

Physical regulation of copper catalyst with a hydrophobic promoter for enhancing CO2 hydrogenation to methanol

The hydrogenation of CO2 to methanol, which is restricted by water products, requires a selective removal of water from the reaction system. Here, we show that physically combining hydrophobic polydivinylbenzene with a copper catalyst supported by silica can increase methanol production and CO2 conversion. Mechanistic investigation reveals that the hydrophobic promoter could hinder the oxidation of copper surface by water, maintaining a small fraction of metallic copper species on the copper surface with abundant Cuδ+, resulting in high activity for the hydrogenation. Such a physically mixed catalyst survives the continuous test for 100h owing to the thermal stability of the polydivinylbenzene promoter.

CuCeOx/CuO Catalyst Derived from the Layered Double Hydroxide Precursor: Catalytic Performance in NO Reduction with CO in the Presence of Water and Oxygen.

Valencies of metal species and lattice defects, such as oxygen vacancies, play a pivotal role in metal oxide-catalyzed reactions. Herein, we report a promising synthetic strategy for preparing CuO-supported CuCeOx catalysts (CuCeOx/CuO) by calcination of a hydrotalcite precursor [Cu6Ce2(OH)16]CO3·nH2O. The structural and chemical properties of catalysts were characterized by XRD, ICP-AES, TEM, TPR, NH3-TPD, XPS, Raman spectroscopy, and N2 adsorption, which revealed that the thermal pretreatment in an oxidative atmosphere caused segregation and reconstitution processes of the precursor, resulting in a mesoporous catalyst consisting of well-dispersed CuO-supported CuCeOx clusters of 1.8-3.2 nm in size with a high population of oxygen vacancies. The as-prepared catalyst shows excellent catalytic performance in the reduction of NO by CO in the absence as well as in the presence of water and oxygen. This behavior is attributed to its high oxygen defect concentration facilitating the interplay of the redox equilibria between Cu2+ and reduced copper species (Cu+/Cu0) and (Ce4+/Ce3+). The high surface population of oxygen vacancies and in situ-generated metallic copper species have been evidenced by Raman spectroscopy and X-ray photoelectron spectroscopy. The layered double hydroxide-derived CuCeOx/CuO also showed good water tolerance and long-term stability. In situ infrared spectroscopy investigations indicated that adsorbed hyponitrite species are the main reaction intermediates of the NO conversion as also corroborated by theoretical simulations.

Electrocatalysts Derived from Copper Complexes Transform CO into C2+ Products Effectively in a Flow Cell.

Electrochemical reactors that electrolytically convert CO2 into higher-value chemicals and fuels often pass a concentrated hydroxide electrolyte across the cathode. This strongly alkaline medium converts the majority of CO2 into unreactive HCO3 - and CO3 2- byproducts rather than into CO2 reduction reaction (CO2RR) products. The electrolysis of CO (instead of CO2 ) does not suffer from this undesirable reaction chemistry because CO does not react with OH- . Moreover, CO can be more readily reduced into products containing two or more carbon atoms (i. e., C2+ products) compared to CO2 . We demonstrate here that an electrocatalyst layer derived from copper phthalocyanine (CuPc) mediates this conversion effectively in a flow cell. This catalyst achieved a 25 % higher selectivity for acetate formation at 200 mA/cm2 than a known state-of-art oxide-derived Cu catalyst tested in the same flow cell. A gas diffusion electrode coated with CuPc electrolyzed CO into C2+ products at high rates of product formation (i. e., current densities ≥200 mA/cm2 ), and at high faradaic efficiencies for C2+ production (FEC2+ ; >70 % at 200 mA/cm2 ). While operando Raman spectroscopy did not reveal evidence of structural changes to the copper molecular complex, X-ray photoelectron spectroscopy suggests that the catalyst undergoes conversion to a metallic copper species during catalysis. Notwithstanding, the ligand environment about the metal still impacts catalysis, which we demonstrated through the study of a homologous CuPc bearing ethoxy substituents. These findings reveal new strategies for using metal complexes for the formation of carbon-neutral chemicals and fuels at industrially relevant conditions.

Copper-dolomite as effective catalyst for glycerol hydrogenolysis to 1,2-propanediol

A series of Cu/dolomite catalysts were synthesized using the impregnation technique, characterized using NH3–TPD, FTIR-Pyridine, XRD, H2-TPR, BET, BJH, FESEM-EDX, and XPS techniques and evaluated in glycerol hydrogenolysis into 1,2-propanediol (1,2-PDO). Remarkably, dolomite support exhibited high acidity, which is, to our knowledge the first acid characteristic revealed among the reported literatures. By doping copper on dolomite support, the acid amount and strength of the catalyst increased. N2O chemisorption analysis suggests that the metallic copper species were well dispersed on dolomite support while the copper surface area increased with copper loading. The formation of metallic copper on dolomite support agreed well with findings derived from XRD and XPS analysis. According to the results of XPS and H2-TPR, metallic copper species were enriched on the grain surfaces of dolomite and not in the bulk. The addition of copper to dolomite ameliorates the redox properties of the catalysts, owing to the reduction at a lower temperature than that of pure CuO and dolomite support. From the catalytic results, 20 wt% Cu/dolomite was the most active catalyst by giving 100% glycerol conversion and 92% selectivity toward 1,2-PDO at 180 ºC, 2 MPa H2 in 6 h reaction time.

Enhanced Stability of Cu Clusters of Low Atomicity against Oxidation. Effect on the Catalytic Redox Process

By a combination of theoretical modeling and XPS and SERS spectroscopic studies, it has been found that it is possible to stabilize metallic copper species under oxidizing reaction conditions by adjusting the atomicity of subnanometer copper clusters. Small Cu5 clusters display low reactivity toward O2 dissociation, being less susceptible to oxidation than larger Cu8 or Cu20 systems. However, in the presence of water this reactivity is strongly enhanced, leading to oxidized Cu5 clusters. In that case, the interaction of Cu5 with atomic O oxygen is weak, favoring recombination and O2 desorption, suggesting an easier transfer of O atoms to other reactant molecules. In contrast, copper clusters of higher atomicity or nanoparticles, such as Cu8 and Cu20, are easily oxidized in the presence of O2, leading to very stable and less reactive O atoms, resulting in low reactivity and selectivity in many oxidation reactions. Altogether, Cu5 clusters are proposed as promising catalysts for catalytic applications where...

Promotional Effect of Water on Direct Dimethyl Ether Synthesis from Carbon Monoxide and Hydrogen Catalyzed by Cu–Zn/Al2O3

Cu–Zn/Al2O3 was prepared by using the sol–gel method and employed for the direct synthesis of dimethyl ether (DME) from synthesis gas (syngas) in a continuous-flow reactor. We studied the effect of water concentration in the feed on the formation rates of various products. With an increase in the amount of water, the formation rate of DME initially increased and then decreased. Co-feeding of 0.09% water was co-fed resulting in the highest formation rate of DME, 486 μmol gcat–1 h–1 (8.0% yield of DME), at 220 °C under 0.9 MPaG. Structural studies by XANES, EXAFS, XPS, and infrared spectroscopies showed that the treatment of the catalyst with 0.66% water at 240 °C under atmospheric pressure resulted in oxidation of the metallic copper species and increased the number of surface Cu+ sites. Combined with the catalytic results, we conclude that the increase in the surface Cu+ sites is responsible for the promotion effect of steam on the DME production.

Hydrogenation of biomass-derived compounds containing a carbonyl group over a copper-based nanocatalyst: Insight into the origin and influence of surface oxygen vacancies

New Mn-containing spinel-supported copper nanocatalysts were directly generated via a Cu–Mn–Al layered double hydroxide precursor route and employed in gas-phase hydrogenation of dimethyl succinate (DMS) to γ-butyrolactone (GBL). It was found that the introduction of manganese into catalyst precursors led to the formation of Mn-containing spinel phases, thereby giving rise to highly dispersive Cu0 nanoparticles and a large number of surface defects (i.e., oxygen vacancies (Ov), Mn2+ species) in reduced catalysts. As-formed copper-based nanocatalysts exhibited exceptional catalytic hydrogenation performance with stability enduring up to 100h. Such high catalytic efficiency could reasonably be attributed to the surface synergism between Mn2+–Ov–Mn2+ defect structures and active metallic copper species, which controlled the key to hydrogenation related to the adsorption of DMS molecules and following activation of carbonyl groups and the dissociation of hydrogen. Most importantly, such copper-based nanocatalysts displayed great potential applicationsin the hydrogenations of other biomass-derived compounds containing carbonyl groups (e.g., acetol, levulinic acid, levulinic acid esters, and furfural). The present strategy enables us to tune the surface structures of catalysts for designing new type of copper-based catalysts with significantly enhanced catalytic performance.

A phenomenological study of the electro-assisted reductive leaching of chalcopyrite

Chalcopyrite can be electro-reduced in acid media to less refractory mineral phases such as chalcocite and metallic copper. This paper presents the effect of current density, pulp density, acidity, flow of oxygen and agitation on the chalcopyrite reduction kinetics. The study was complemented with SEM and X-ray characterization techniques in order to understand the phenomena taking place in an electrolytic reactor with anionic separator. The results revealed that when the pulp density is increased (>10g/L), the main problem of the electro-assisted reduction of chalcopyrite is observed, i.e. chalcopyrite reduction kinetics is considerably hindered. This phenomenon was found to be related to the chemical interaction of the hydrogen sulfide (released during the chalcopyrite reduction) with the oxygen present in the electroleaching system. Elemental sulfur is the product of such chemical interaction, which in the same case as in the conventional oxidative processes, promotes the passivation of chalcopyrite due to the fact that it is a highly non-conductive and hydrophobic species. On the other hand, the electrochemical experiments revealed that chalcopyrite reduction kinetics is dependent on the acidity and agitation rate, showing that chalcopyrite reduction is controlled by the diffusion of protons in the solid-liquid interface. The solid residue characterization revealed that the metallic copper species (product of the chalcopyrite reduction), is very reactive with air (after filtering), producing copper oxide or copper sulfate with and without rinsing, respectively. These results were corroborated by electrochemical techniques, EDS and X-Ray diffraction.

Bioactive and biocompatible copper containing glass-ceramics with remarkable antibacterial properties and high cell viability designed for future in vivo trials.

In the present study our interest is focused on finding the efficiency of 60SiO2·(32 - x)CaO·8P2O5·xCuO (mol%) glass-ceramics, with 0 ≤ x ≤ 4 mol%, in terms of bioactivity, biocompatibility, antibacterial properties and cell viability in order to determine the most appropriate composition for their further use in in vivo trials. The sol-gel synthesized samples show a preponderantly amorphous structure with a few crystallization centers associated with the formation of an apatite and calcium carbonate crystalline phases. The Fourier Transform Infrared (FT-IR) spectra revealed slightly modified absorption bands due to the addition of copper oxide, while the information derived from the measurements performed by transmission electron microscopy, UV-vis and electron paramagnetic resonance spectroscopy showed the presence of ions and metallic copper species. X-Ray photoelectron spectroscopic analysis indicated the presence of copper metallic species, in a reduced amount, only on the sample surface with the highest Cu content. Regarding in vitro assessment of bioactivity, the results obtained by X-ray diffraction, FT-IR spectroscopy and scanning electron microscopy, demonstrated the formation of a calcium phosphate layer on all investigated sample surfaces. The inhibitory effect of the investigated samples was more significant on the Pseudomonas aeruginosa than the Staphylococcus aureus strain, the sample with the lowest concentration of copper oxide (0.5 mol%) being also the most efficient in both bacterial cultures. This sample also exhibits a very good bactericidal activity, for the other samples it was necessary to use a higher quantity to inhibit and kill the bacterial species. The secondary structure of adsorbed albumin presents few minor changes, indicating the biocompatibility of the glass-ceramics. The cell viability assay shows a good proliferation rate on samples with 0.5 and 1.5 mol% CuO, although all glass-ceramic samples exhibited a good in vivo tolerance.

Methanol dehydrogenation over Cu/SiO<SUB align="right">2 catalysts

Methanol dehydrogenation was carried out over impregnated Cu/SiO2 catalysts. It was found that the temperature of the catalyst's preliminary reduction by hydrogen could play a dramatic role. The interaction of copper ions with silica during the reduction at ≥300°C was observed. Metallic copper species obtained at low reduction temperature were found to be responsible for the methyl formate formation. The catalysts containing copper in the oxidised state showed low selectivity because of a side reaction of coke formation and were deactivated rapidly. The optimal temperature of reduction was found to be 250°C.

Gas phase hydrogenation of dimethyl-1,4-cyclohexane dicarboxylate over highly dispersed and stable supported copper-based catalysts

Highly dispersed and stable supported copper-based catalysts for gas-phase hydrogenation of dimethyl 1,4-cyclohexane dicarboxylate (DMCD) to 1,4-cyclohexane dimethanol (CHDM) were synthesized from a single-source Cu–Zn–Al layered double hydroxide (LDH) precursor. The catalysts were characterized by X-ray diffraction (XRD), hydrogen temperature-programmed reduction (H2-TPR), X-ray photoelectron spectroscopy (XPS), N2 sorption, H2-N2O titration and transmission electron microscopy (TEM) techniques. The results revealed that calcination temperature for LDH precursors significantly affected the composition, structure, chemical states, and catalytic performance of copper-based catalysts. In the gas-phase selective hydrogenation of DMCD, a high CHDM yield of 95.6% was achieved over copper-based catalyst obtained at calcination temperature of 600°C. Compared with the conventional Cu/ZnO/Al2O3 and Cr2O3 promoted copper-based catalysts, LDH-derived Cu-based catalysts exhibited higher catalytic performance, which was mainly attributed to the presence of highly dispersed and stable active metallic copper species, as well as a reasonable amount of Cu+ species facilitating the activation of ester groups of reactants.

Aluminum‐Doped Zirconia‐Supported Copper Nanocatalysts: Surface Synergistic Catalytic Effects in the Gas‐Phase Hydrogenation of Esters

AbstractThe efficient gas‐phase selective hydrogenation of a series of esters to the corresponding alcohols was achieved over well‐dispersed aluminum‐doped zirconia‐supported copper nanocatalysts (Cu/Al‐ZrO2), which were prepared through a homogeneous coprecipitation route in the presence of cetyl trimethyl ammonium bromide. The characterization revealed that the structure and catalytic performance of Cu/Al‐ZrO2 nanocatalysts were profoundly affected by the addition of Al. Compared with the Al‐free catalyst, Al‐doped materials had higher specific surface areas and smaller copper nanoparticles. In particular, the results confirmed that the incorporation of Al into the ZrO2 framework could form tetrahedrally coordinated Al3+ species, leading to the improvement of metal dispersion and the formation of more surface Lewis acid sites. In the gas‐phase selective hydrogenation of dimethyl oxalate (DMO) to ethylene glycol (EG), 100 % DMO conversion, 97.1 % EG selectivity, and a high turnover frequency of 16.9 h−1 were achieved over Cu/Al‐ZrO2 catalyst with a Al/(Cu+Zr+Al) mass ratio of 0.1. The high efficiency of Cu/Al‐ZrO2 catalysts in DMO hydrogenation was attributed mainly to the surface synergistic catalytic effect between highly dispersed metallic copper species and strong Lewis acid sites, which promoted the hydrogenation reaction related to the ester groups, unlike the single case of Cu+Cu0 synergy reported previously that was found to control the extent of hydrogenation. The obtained catalysts displayed excellent catalytic performance in the gas‐phase hydrogenation of other esters including dimethyl succinate, dimethyl maleate, dimethyl adipate, and 1,4‐cycolhexane dicarboxylate.

Highly-Dispersed Copper-Based Catalysts from Cu–Zn–Al Layered Double Hydroxide Precursor for Gas-Phase Hydrogenation of Dimethyl Oxalate to Ethylene Glycol

The highly-dispersed copper-based catalysts for the gas-phase hydrogenation of dimethyl oxalate to ethylene glycol (EG) were prepared from a Cu–Zn–Al layered double hydroxide (LDH) precursor. Powder X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), N2 adsorption–desorption, H2 temperature programmed reduction (H2-TPR) and H2–N2O titration indicated that the composition, texture, and structure of resulting copper-based catalysts were profoundly affected by the calcination temperature of LDH precursor. Moreover, the as-synthesized catalyst calcined at 600 °C was found to exhibit a superior catalytic hydrogenation performance with an EG yield of 94.7 % to the other catalysts calcined at 500 and 700 °C, which should be mainly attributed to the presence of the highly-dispersed active metallic copper species over metal oxide matrix.

Physicochemical properties of a plate-type copper-based catalyst, prepared on an aluminum plate by electroless plating, for steam reforming of methanol and CO shift reaction

Abstract A plate-type copper-based catalyst was prepared by electroless plating on an aluminum substrate. Its physicochemical properties were measured to examine the relation to methanol reforming and CO shift performances. The catalytic activity of the plated copper-based catalyst was improved more by oxidation treatment in air than by reduction treatment prior to the reaction. In this case, the oxidation treatment caused the zinc located in the bulk layer to migrate to the surface and form a CuZn alloy-like compound. While, the reduction treatment made only a small amount of zinc migration, so the CuZn alloy was barely formed. The surface area of metallic copper component on the catalyst was increased by the reduction treatment, and with the number of reductions; however, there was no relationship between such metallic surface area and CO shift activity. The valence of the copper species at the surface layer was metallic after reduction treatment and cationic after oxidation treatment or reforming reaction. It was found that the presence of metallic copper species on the plated catalyst hardly contributes to the formation of active site. The formate species, which are considered as the intermediate of reforming and CO shift reaction, were adsorbed on the catalyst in the form of a monodentate-type or a bridge-type (and/or bidentate-type). On the surface that experienced oxidation treatment, the proportion of the monodentate-type formate group was higher. It was inferred that the formation of a CuZn alloy-like compound accelerates the increase of monodentate-type formate group and contributes to the improvement of catalytic activity.

Physicochemical and Reactivity Studies on Polystyrene‐Cu/Al2O3 Catalysts: An Attempt to Control Copper Dispersion

The influence of coating of 5.0 (w/w%) Cu/γ‐Al2O3 catalyst by different ratios of polystyrene on the physicochemical and textural properties was studied. The physicochemical and textural properties of polystyrene‐`Cu/γ‐Al2O3 catalysts were investigated by N2 adsorption, O2 chemisorption, FTIR, XRD, TEM, and SEM. In addition, the kinetics of H2O2 decomposition as a model redox reaction over polymer coated and uncoated catalysts was investigated. The highest activity was achieved by 0.06 wt% polystyrene‐5.0Cu/γ‐Al2O3 catalyst. The parent 5.0Cu/γ‐Al2O3 catalyst showed auto‐catalytic first order mechanism, which was subjected to a pronounced modification to a simple first order one upon coating by polystyrene. This modification in the mechanism was accompanied with an increase in the apparent activation energy of the reaction. The observed high activity of 0.06 wt% polystyrene‐5.0Cu/γ‐Al2O3 catalyst was attributed to the role of polymer in enhancement of the degree of dispersion of the surface copper. However, the modification in kinetics of the reaction was attributed to the difference in the nature of Cu active sites namely, the polymer protected the metallic copper species on the surface of γ‐Al2O3 support against possible oxidation to copper sub‐oxides and/or that polymer might change the hydrophilic properties of the reaction media.

Catalytic properties of La2CuO4 in the CO + NO reaction

La2CuO4 is an active catalyst for the reduction of NO by CO. Under reaction conditions, the catalyst exhibits an activation which results in a lowering of the light‐off temperature by 80°C. XRD, TEM and EDX analysis carried out after the catalytic test indicate that the mixed oxide has been reduced to form a La2O3, Cu binary system. It seems that metallic copper species are the most active sites in the CO + NO reaction.